From Arabic رُبْع (rubʿ).
robo (n class, plural robo)
1) one-fourth; a quarter
Some fractions have especial names, generally taken from the Arabic, but those do not follow standard patterns. Otherwise, those with 1 as the numerator are expressed with the n class noun sehemu (“part”) followed by the ordinal of the number in question, so "one-fifth" is "a fifth part". Those with numbers greater than 1 in the numerator are expressed with kwa, saying the denominator followed by the word kwa followed by the numerator.
Robo comes from Arabic رُبْع (rubʿ "one fourth; quarter"), from the root ر ب ع (r-b-ʿ); compare أَرْبَعَة (ʾarbaʿa) and Hebrew אַרְבַּע (arbá), Maltese erbgħa, all meaning "four".